Homebrew "touch" (flex) sensors (N&V June 2009)

Did anyone else notice John Iovine's article "How to Make Bi-Directional Flex Sensors" in the June issue of Nuts & Volts
magazine?
While resistive flex sensors have been available for several years, they haven't been cheap, and the shape options have been a bit limited. Mr. Iovine's article suggests that these could be home-built, shaped-to-order, and inexpensive enough that that many of these could be added to a robot without blowing a hobbyist's budget.
The sensor design seemed simple enough: a sandwich made up of a strip of thin and flexible plastic (stiffener), two strips of thin copper surrounding a resistive layer of some sort, and shrink-wrap tubing to keep the ham and cheese... er, "conductive layers" from squishing out. The resistive layer might be tricky (_cloth_??), but I was fairly sure I had some black conductive bags around here "somewhere" (translation: only a day or two of digging <grin!>).
And it looks like Mr. Iovine's approach would make it possible to design (say) a flex sensor that could report, with reasonable accuracy, human finger flexing by using a "club sandwich" flex/copper/resistance design:
flex: =============================Cu1: ---------- Res1: oooooooooo Cu2: -------------------- Res2: oooooooooo Cu3: -------------------- Res3: oooooooooo Cu4: ----------
Layer Cu3 might look like this from above (not to scale) if you wanted all terminals to be at one end of the sensor:
-----------------------------------+ ----------------+R2R2R2R2 | |R2R2R2R2 | |R2R2R2R2 | |R2R2R2R2 | |R2R2R2R2 | |R2R2R2R2 | +------------------+
So I went exploring Saturday afternoon. It looks like I need a new set of places to look for oddments: the two big local art/craft stores (Micheals, Ben Franklin) could provide sheet copper for "embossing", but two thicknesses of their 36ga stuff seemed a bit stiff for small robot sensors. No luck on flexible plastic.
Home Depot had some rolls of copper sheet ("roof flashing") that seemed a bit thinner, but I didn't need 12"x20' of it, not at $33/roll. No plastic there, either. Fortunately we have one "real" hardware store here in Richmond -- Pleasants -- but I foolishly didn't start with them, and now I'm temporarily sidetracked.
The article mentions that parts and pre-built sensors can be ordered from the Images Scientific Instruments 'web site:
Images Scientific Instruments http://www.imagesco.com/sensors/flex-sensor.html
(Based on the prices quoted there, it looks like $12 would purchase enough material for 12 0.3"x5" sensor strips.)
Has anyone else tried making one of these? If so, what did you use, and what were your results?
Frank McKenney -- The vice of the modern notion of mental progress is that it is always concerned with the breaking of bonds, the effacing of boundaries, the casting away of dogmas. But if there is such a thing as mental growth, it must mean the growth into more and more definite dogmas. The human brain is a machine for coming to conclusions; if it cannot come to conclusions it is rusty. -- G.K. Chesterton: Concluding Remarks on the Importance of Orthodoxy (1905) -- Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 18 Aug 2009 08:47:23 -0500, Frnak McKenney
[snippety snip]

It's on the "to do" list (somewhere)
Small Parts http://www.smallparts.com/ has copper sheet but I don't see any that has acetate backing but that doesn't seem to be a necessary characteristic.
And, of course, McMaster-Carr (which seems to have pretty much everything) has copper foil in various form-factors. http://www.mcmaster.com/#copper-foil/8py95
(BTW - stick a space character after your double-dash for the "standard" usenet sig delimiter. I.e., "-- " vice "--")
--
Rich Webb Norfolk, VA

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi, Rich. Thanks for the response.

Thank you. <grin!>

Hm. Wonder if that's the same "somewhere" where I stored my black electrostatic bags, the "safe place where I can find it when I need it"?

No. I'm not exactly sure why the author specified "laminated" copper, since the laminate's insulating properties aren't used, and the separate strip of acetate (or whatever) provides the needed stiffness/springiness. Also not sure what happens to laminated copper when you solder to it: does it separate?
You did prompt me to dig up the relationship between PC board copper in "ounces" and sheet metal thickness: 1oz ~= .0012in

Thanks.
The copper tape listed in McMaster-Carr is .0035", which would make it roughly equivalent to "3oz copper" on a printed circuit board. With conductive adhesive, it's only $13.03 for 1/2" wide, $8 for 1/4" wide; wonder how it would change the sensor's characteristics if you put the adhesive side toward the resistive material? Presumably that wouold mean better electrical contact, but in the original (glueless) design the copper and plastic can slide a bit against one another; I wonder what happens when the three layers are "glued" together and it _can't_ slide?
Copper mesh (screen) would have a different "contact pattern" than foil/sheet: it would press against the resistive material only at the acreen's "peaks". Fewer points of contact, but more pressure at those points... hard to guess how that would affect the sensor.
Oh, and then there's copper 101, copper 110, and beryllium copper. I seem to recall beryllium-copper being used in springs; wonder if using that would let us omit the acetate strip?

Thanks for reminding me. It's not easy to see the extra space sometimes... most times... okay, ever. <grin!>
Frank
--
...[S]tigmas are the corollaries of values. If work, indpendence,
responsibility, respectability are valued, then their converse must
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Frnak McKenney wrote:

Haven't seen John's article but I'd opt for foil tape, a fairly common find at craft stores (but maybe not Michael's). Widths are from 1/4" to maybe 1", with thicknesses from 1 to 2 mil or so.
Masterfoil is a popular brand. It's used most in stained glass work, but it has many uses. I've made *capacitative* sensors out of these kinds of tapes, but never resistive. Might be an interesting experiment.
-- Gordon
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi, Gordon. Thanks for joining in.
On Tue, 18 Aug 2009 10:24:47 -0700, Gordon McComb

Hm. Stained glass... that sparks a memory. I may have a roll of that around here... um, "somewhere". Perhaps if I look for the electrostatic bags first...
Frank
P.S. Thank you (and Rich) for not pointing out that I misspelled the magazine name. It should have been "June 2009 SERVO" rather than "June 2009 Nuts & Volts". Sorry about that.
--
It is the business of education to implant insight and respon-
sibility. It must turn irresponsible opinion into responsible
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.